Support extension-based file-handlers for packaged apps.
This rolls forward r197909 but without the windows breakage.
Previously, file_handlers could only specify the file types they
accepted by MIME type. This limited apps to only accepting types known
to the local operating system or accepting any unknown type. This change
allows file_handlers to restrict types by file extension.
BUG=167950
TEST=browser_tests PlatformAppBrowserTest.Launch*
Review URL: https://chromiumcodereview.appspot.com/
14887004
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@198091 0039d316-1c4b-4281-b951-d872f2087c98